1. Field of the Invention
This invention relates to a system and method for controlling the thickness of a coating applied to a moving substrate, more particularly for controlling the thickness and distribution of a zinc coating applied to a steel substrate in a "hot dip" galvanizing process.
2. Prior Art
In a "hot dip" galvanizing process, a moving substrate such as steel is coated with a material such as molten zinc by feeding the substrate through a coating bath. The substrate emerges from the bath along a generally vertical feed path with molten coating material deposited on its surfaces. The coating process must be controlled to assure a substantially uniform coating thickness on the resulting product. Such control avoids wasteful deposition of excessively thick coatings, and assures that the coated substrate will perform in a predictable and desired manner in such handling processes as coiling, stacking, and shipment, and in such fabricating processes as die forming and welding. Coating thickness depends on factors that include the speed at which the substrate proceeds through the coating line, which is typically variable. A relatively high substrate velocity is usually maintained during most of a coating run, but will be reduced from time to time to permit the attachment of a new source of substrate to a source nearing depletion.
While the basic thickness of a desired coating can be modified by the line speed, coating thickness and distribution also can be modified after a coated substrate has emerged from the coating bath. Control system have included such devices as rolls that wring the coated substrate to the proper coating thickness, and fluid devices, which do not engage the substrate, but rather control substrate coating thickness by directing controlled streams of pressurized fluid toward the coated substrate. There is a trend in the galvanizing industry to convert to coating control methods and systems that utilize some form of a fluid knife to avoid mechanical problems associated with wringers. The advantages of such knives over coating rolls have been clearly demonstrated, and more recently the advantages of air over steam as a knife fluid has also been shown. Generally, the air systems use high volume, low pressure air for reasons related to efficiency, noise and simplicity. The state of the art now permits air knives to control a great range of coating thicknesses over a broad range of line speeds on large variations of strip width and gauge.
Air requirements for the knives vary both with the substrate line speed and the width of the substrate being coated, and with the coating thickness desired. By way of example, under various operating conditions, opposing air knives may require as much as 8000 cubic feet per minute (CFM) or as little as 200 CFM or less. A blower capable of supplying the maximum quantity of air can be reduced in speed, but not below its slowest stable point of operation. Even at that point the blower output may be many times more than that desired for the particular knife application.
Past solutions to this problem have included simple venting to the atmosphere or bleeding off of unwanted air volume. Those solutions are inefficient, and such methods have tended to be erratic and unpredictable with complex controlling apparatus. Moreover, the same conditions that require very low quantities of air from the coating knife, namely thick coatings on narrow stock, have typically required a slower line speed than otherwise desired, because the thicker zinc coating freezes slowly and must be given adequate time to solidify before reaching conveying rollers outside the bath. Thus, the slow speed coating operation presents a two-fold problem of difficult blower control complicated by a long coating solidification period.